A wind turbine known in the art comprises a tapered wind turbine tower and a wind turbine nacelle positioned on top of the tower. A wind turbine rotor with a number of wind turbine blades is connected to the nacelle through a low speed shaft, which extends out of the nacelle front as illustrated on FIG. 1.
The pitching of wind turbine blades is commonly done with a hydraulic system based on electrical powered oil pump, proportional valve and hydraulic cylinder acting on the blade. A much more direct way is to let an electrical motor act directly on the blade. Pitch systems has been made with electromechanical activation. One known way is to make the blade rotation with a geared motor, which rotate the blade by an open gear. Another known way is to have a linear activator (spindle and motor) to replace the function of the hydraulic cylinder. Both solutions have the inhered disadvantage, which is the motion of moving contact in gears or in treats. This means risk of wear, which can limit the life of the elements.
A contact free pitch motor could be advantageous. Such can be imagined. Think of an electrical motor, where the case is flanged to the hub and the blade is mounted on the rotor. If the motor is dominantly disk-shaped-like and merged with a blade bearing, say the motor has almost the same diameter as the blade bearing, a motorized slewing unit is made.
A motor for such design will have a large diameter and will naturally be without centre. The motor will look like a slewing ring, where the one ring is the rotor and the other is the stator. An example of this is disclosed in the international patent application WO 2005/019642 A1, where the rotor of a direct drive motor is attached more or less directly to a wind turbine blade and the stator is connected to the hub.
In this design, the direct drive motor has to handle the full blade torque moment. This demands a motor with very high torque and very low speed, which results in a large and expensive motor.
WO 2005/019642 A1 further disclose that the direct drive motor could act on the blade through a bull gear. But this solution has the implications, that to reduce the torque the bull gear ring—on which the pinion acts—has to be as large as possible, which results in limited space for a direct drive motor concentric to the pinion.
Say the bull gear is dentations of the inner ring of the blade bearing and almost in plane with the inner of the blade and hub. Then the motor cannot be much larger than the pinion. In such case the use of a direct motor makes no sense. Decreasing the bull gear diameter makes space for a larger direct motor, but reduces the gear ratio; hence the direct motor must have a larger torque capacity. The solution with a pinion and bull gear is contradictive to their statement about the benefit of the direct motor.
The use of a pinion also requires separate bearings for both the pinion and the direct motor. Most disadvantageously is the problem of having one large pinion in mesh with the bull gear. The tooth, which is in mesh at the dominant tip angle will be loaded frequently, which will cause wear and fatigue considerations.
An object of the invention is therefore to provide for an advantageous technique for controlling the blade of a wind turbine rotor, which do not include the above mentioned disadvantages.